SOI semiconductor device capable of preventing floating body effect

ABSTRACT

The present invention provides an SOI device preventing the floating body effect, and a method for manufacturing the same. Disclosed is a method comprising the steps of: forming an isolation layer on a first silicon substrate; forming a conductive layer on the isolation layer and the first silicon substrate; forming a buried insulating layer on the conductive layer; bonding the second silicon substrate so as to contact with the buried insulating layer; exposing the isolation layer by removing backside of the first silicon substrate by selected thickness thereby defining a semiconductor layer; forming a transistor by forming a gate electrode, a source region and a drain region at selected portions of the semiconductor layer; etching a selected portion of the isolation layer so as to expose the conductive layer; and forming a body electrode to be contacted with the conductive layer within the isolation layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a silicon-on-insulator (SOI) semiconductor device, and more particularly, to an SOI semiconductor device capable of preventing floating body effect of the SOI device and method of manufacturing the same.

2. Description of the Related Art

As semiconductor devices have high performance, the SOI substrate (hereinafter “SOI device”) has been suggested remarkably instead of silicon substrate made of bulk silicon. The SOI substrate comprises a handling wafer as a supporting part, a buried insulating layer and a semiconductor layer on which a device is formed later. An SOI device formed on an SOI substrate is completely isolated by a buried oxide layer and field oxide layer, and a junction capacitance is reduced, thereby achieving low power consumption and fast operation.

FIG. 1 is a cross-sectional view showing a conventional SOI device in which a transistor is formed. As shown in the drawing, an SOI substrate 10 comprising a handling wafer 11, a buried insulating layer 12 and a semiconductor layer 13, is provided. A field oxide layer 14 is formed on a selected portion of the semiconductor layer 13 of the SOI substrate 10 thereby defining an active region. A bottom of the field oxide layer 14 is in contact with the buried insulating layer 12. A gate electrode 16 having a gate insulating layer 15 is formed at a selected portion of the semiconductor layer 13, and a sidewall spacer 17 is formed of an insulating layer at both sidewalls of the gate electrode 16. Junction regions 18 a, 18 b are formed at the semiconductor layer 13 of both sidewalls of the gate electrode 16. Bottoms of the junction regions 18 a, 18 b are in contact with the buried insulating layer 12.

In a transistor formed on such SOI substrate, the junction regions 18 a, 18 b are in contact with the buried insulating layer 12 and the junction capacitance is lower than the bulk silicon device, thereby performing fast operation. Further, if thickness of the semiconductor layer 13 is below 100 nm, on-current of transistor can be increased.

However, if the semiconductor layer 13 on which a transistor is formed later is separated by the field oxide layer 14 and the buried insulating layer 12, and the semiconductor layer 13 is formed of thin film, then potential within a channel region is higher than that within a conventional MOS transistor when the channel layer is completely depleted. Moreover, a potential barrier between source region and the channel region is lowered. Holes generated by the impact ions of the depletion layer at the drain side, are temporarily stored in the channel region. By doing so, potential in the channel region is raised and electrons are rapidly injected from the source region to the channel region. Thus, the floating body effect, i.e. a decrease of voltage between the source and the drain regions, occurs. The floating body effect causes malfunction of semiconductor device.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an SOI device capable of forming thin semiconductor layer and also preventing the floating body effect, and a method for manufacturing the same.

In one aspect, the present invention provides an SOI device comprising: a substrate; a buried insulating layer formed on the substrate; a conductive layer formed on the buried insulating layer; a semiconductor layer formed on the conductive layer; an isolation layer formed on a selected portion of the semiconductor layer and defining an active region; a transistor comprising a gate electrode formed on a selected portion of the active region of the semiconductor layer, and source and drain regions formed at the active regions of both sides of the gate electrode; and a body electrode formed to be contacted with the conductive layer within the isolation layer, and applying a selected degree of voltage to the conductive layer.

In another aspect, the present invention provides a method for fabricating SOI device comprising the steps of: forming an isolation layer on a first silicon substrate; forming a conductive layer on the isolation layer and the first silicon substrate; forming a buried insulating layer on the conductive layer; bonding the second silicon substrate so as to contact with the buried insulating layer; exposing the isolation layer by removing backside of the first silicon substrate by selected thickness thereby defining a semiconductor layer; forming a transistor by forming a gate electrode, a source region and a drain region at selected portions of the semiconductor layer; etching a selected portion of the isolation layer so as to expose the conductive layer; and forming a body electrode to be contacted with the conductive layer within the isolation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects advantages will be better understood from the following detailed description of the invention with reference to the drawings, in which:

FIG. 1 is a cross-sectional view showing a conventional SOI device; and

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, and 2I are cross-sectional views for showing the method for fabricating a SOI device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment of this invention will be explained in detail with reference to the accompanying drawings.

Referring to FIG. 2A, a photoresist pattern 21 is formed at a selected portion of a first silicon substrate 20 so that an isolation region to be formed later on is exposed.

Referring to FIG. 2B, by using the photoresist pattern 21 as a mask, the exposed first silicon substrate is etched to a selected depth thereby forming a trench 22. The thickness of a semiconductor layer to which a device is formed is selected according to the depth of the trench 22. In this embodiment, the thickness of trench 22 is set below 100 nm approximately.

As shown in FIG. 2C, a thermal oxide layer 23 is formed by a known thermal oxidation process on the first silicon substrate 20 in which the trench 22 is formed. An oxide layer 24 for trench-filling is formed with thickness sufficient to fill the trench 22 according to the chemical vapor deposition(CVD) method.

Next, referring to FIG. 2D, the oxide layer 24 for trench-filling is etched-back or chemical mechanical polished until a surface of the first silicon substrate 20 is exposed, thereby forming a trench isolating layer 25 within the trench 22.

As shown in FIG. 2E, a conductive layer 26 for preventing the floating body effect is formed with thickness of below 100 nm on the first silicon substrate 20 in which the trench 22 is formed. The conductive layer 26 is formed of, for example a doped-silicon layer or a doped-polysilicon layer. At this time, the conductive layer 26 is formed by the method of LPCVD (low pressure chemical vapor deposition), PECVD (plasma enhanced chemical vapor deposition), ECR (electron cyclone resonator), APCVD (atmosphere pressure chemical vapor deposition) or photo-CVD (chemical vapor deposition). Further, when the conductive layer is formed of the doped-silicon layer or the doped-polysilicon layer, dopant can be injected at the same time the conductive layer 26 is deposited. Also, the conductive layer 26 can be formed of intrinsic silicon layer or intrinsic polysilicon layer, and then the POCI₃ doping or impurities can be further injected. At this time, the type of the impurities injected to the conductive layer 26 is preferably opposite to the type of an SOI device to be formed later on. For instance, if an NMOS is planned to be formed, P type impurities, e.g., B ions, are injected into the conductive layer 26, and if a PMOS is planned to be formed, N type impurities, e.g., P ions, are injected into the conductive layer 26.

As shown in FIG. 2F, a buried insulating layer 27 is deposited on the conductive layer 26 according to the CVD method or the thermal oxidation process.

Next, as shown in FIG. 2G, one face of a second silicon substrate 30 for handling is bonded with the buried insulating layer 27 of the first silicon substrate 20. The second silicon substrate 30 and the first silicon substrate 20 are thermally attached at a selected temperature.

As shown in FIG. 2H, the first silicon substrate 20 is grinded and polished to expose the surface of trench isolating layer 25, thereby forming a semiconductor layer 200. At this time, an active region is defined in the semiconductor layer 200 by the trench isolating layer 25. Therefore, an SOI substrate is completed.

Afterward, referring to FIG. 21, a gate oxide layer 31 and a gate electrode 32 are formed at a selected portion of the semiconductor layer 200. At this time, the gate electrode 32 is formed of a doped-polysilicon layer. Impurities are injected into the semiconductor layer 200 between the gate electrode 32 and the trench isolating layer 25 thereby forming source and drain regions 34 a, 34 b. Afterward, a selected portion of the trench isolating layer 25 is etched so that a selected portion of the conductive layer 26 is exposed. Next, a body electrode 35 is formed in the trench isolating layer 25 so as to contact with the exposed conductive layer 26. Herein, the conductive layer 26 contacts the semiconductor layer 200, thereby preventing a floating of the semiconductor layer 200.

Operation of the SOI device is as follows.

When voltage above threshold voltage is applied to the gate electrode 32, current flows between the source and the drain regions 34 a, 34 b. At this time, energy of electrons (in case of NMOS device) flowing in the channel increases by the voltage applied to the drain region 34 b. Thus, the electrons having high energy crash with silicon lattice at the drain region 34 b, thereby creating electrons and holes due to impact ionization. At this time, the newly created electrons flow into the drain region according to the electric field, and holes are gathered from the channel region to the source region 34 a having lower potential. However, in the present invention, since the semiconductor layer 200 is contacted with the conductive layer 26 to which a selected voltage is applied, the potential of a channel region which is formed at the semiconductor layer by the conductive layer 26 is adjusted. That is to say, since a lower voltage such as ground voltage is continuously applied to the body electrode 35, holes (minority of carriers) occurring when the transistor is turned on, are discharged toward the body electrode 35, thereby preventing the body floating effect.

As discussed above, according to the present invention, there is formed the conductive layer between the semiconductor layer and the buried insulating layer so as to adjust the potential of the channel layer. Because the potential of the channel region is adjustable, the floating body effect is completely prevented, even though the semiconductor layer on which an SOI device is formed later is formed of a thin film.

While the present invention has been described with reference to certain preferred embodiment, various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of the present invention. 

What is claimed is:
 1. A silicon-on-insulator (SOI) semiconductor device comprising: a substrate; a buried insulating layer formed on the substrate; a conductive layer formed on the buried insulating layer; a semiconductor layer formed on the conductive layer; an isolation layer of a trench type formed within a selected portion of the semiconductor layer to contact the conductive layer and define an active region; a transistor comprising a gate electrode formed on a selected portion of the active region of the semiconductor layer, and source and drain regions formed at the active regions of both sides of the gate electrode; and a body electrode formed within the isolation layer to be in contact with the conductive layer, and applying a selected degree of voltage to the conductive layer.
 2. The SOI semiconductor device of claim 1, wherein the conductive layer is formed of a doped-silicon layer or a doped-polysilicon layer.
 3. The SOI semiconductor device of claim 1, wherein a type of impurities injected into the conductive layer is opposite to that of impurities of the source and drain regions of the transistor.
 4. The SOI semiconductor device of claim 1, wherein a thickness of the semiconductor layer is below 100 nm. 